Sedimentation Processes on Intertidal Areas of the Lagoon of Venice: Identification of Exceptional Flood Events (Acqua Alta) Using Radionuclides

Abstract

Particular atmospheric conditions produce frequent storm surges in the Lagoon of Venice, locally called "acqua alta": the highest event of this kind was registered in 1966. The process became of some importance in the last 100 years, when man-made subsidence caused a gradual sinking of the town and lagoon's bed. Four cores were collected in a range of intertidal environments of the Northern lagoon to undertake radionuclide studies using profiles of natural 210Pb (using the Constant Rate of Supply and Constant Initial Concentration models) and anthropogenic 137Cs. The best agreement between the three dating methods was found at San Giacomo, an eroding saltmarsh at the edge of a navigation channel, with a sedimentation rate ranging from 0.22 to 0.29 cm yr-1. This site recorded the flood of 1966 as a characteristic break in the 210Pbex profile, as dated by the CRS model. For the Cona tidal flat, both the CRS model and the position of the Cs peak-marker gave similar accretion rates, 0.16 and 0.18 cm yr-1. However, two different CIC accumulation rates were calculated, 0.29 cm yr-1 for the deepest section of the core and 0.17 cm yr-1 for the uppermost part. The break in the 210Pbex profile, again corresponds to the flood of 1966. The effects of subsidence were recorded as an increase in accumulation rate between 1910 and 1931, when there were up to 15 floods per year (1926). Higher sedimentation took place during the period 1958-1973, when years with over than 50 flood events were frequent. The maximum deposition rate (0.43 cm yr-1) occurred again around 1967, consistent with the record of the exceptional flood, if the accuracy of the dating is taken into account. The sedimentation rates calculated for the two other mudflats, Rosa and Saline, were more problematic to interpret because of downcore mixing and/or the occurrence of reducing conditions.